1. Field of the Invention
The present invention relates generally to optical transmitters for converting electrical data pulses to optical data pulses by intensity modulating a continuous wave optical carrier frequency signal, and more particularly to an optical clock modulator for use in an optical transmitter for modulating the optical carrier frequency signal with an electrical clock signal.
2. Description of the Related Art
Optical carrier-suppressed return-to-zero (CS-RZ) modulation is known as an excellent technique tolerant of waveform distortion caused by combined effects of wavelength dispersionand non-linear effects (self-phase modulation and cross-phase modulation, in particular), as described in an article titled “320 Gbps (8×40 Gbps) WDM transmission over 367 km with 120 km repeater spacing using carrier-suppressed return-to-zero format”, Y. Miyamoto et al., Electronics Letters, Vol. 35, No. 23, pp. 2041-2042, 1999. FIG. 1 shows the structure of the optical transmitter of CS-RZ format described in the Miyamoto et al article. The optical transmitter is comprised of a clock modulator 1 and a data modulator 2. The prior art clock modulator 1 includes an optical intensity modulator 3 and an electrical clock source 4 which generates a clock signal at a frequency which is one half of the frequency of a fundamental sinusoidal component of a data pulse supplied to the data modulator 2. For charp-less clock modulation in which phase inversions occur at bit intervals, the intensity modulator 3 operates in a push-pull mode to perform a modulation on a continuous-wave carrier frequency optical signal from a light source 6 with dual-phase clock signals, one of which is supplied direct from the clock source 4 and the other through a π-radian phase shifter 5. The output signal of the clock modulator 1, after being amplified by optical amplifier 7, undergoes data modulation through an intensity modulator 8 which operates in a push-pull mode using dual-phase non-return-to-zero (NRZ) data pulses, one being supplied direct from an external data source 9 and the other through a π-radian phase shifter 10. As a result of the push-pull operation, the output of the intensity modulator 8 is an optical carrier-suppressed return-to-zero (CS-RZ) signal, which is boosted by optical amplifier 11 for transmission to a receive site over an optical link.
Although the prior art CS-RZ format has a superior nonlinear tolerance for self-phase modulation enabling 1.4 dB higher power transmission compared to RZ signal format, there is still a need to provide a robust optical transmitter in terms of non-linearity tolerance and narrow spectral bandwidth for wavelength division multiplexing. In addition, the prior art CS-RZ optical transmitter requires delicate fine tuning of the π-phase shifter 5 for precisely adjusting the phase difference between the two clock signals and precisely adjusting their amplitudes and bias voltage supplied to the intensity modulator 3. Thus, it is desirable to eliminate the need for the delicate tuning of the clock modulator.